It is, in telecommunications, often necessary to characterize a channel between transmitter and receiver so that its capacity can be determined and/or to troubleshoot problems.
One conventional method of characterizing said channel is to send a test signal and subsequently measure its response. If active equipment is connected to both ends of the channel, it is possible to perform transmission measurements. Reflection measurements, on the other hand, can be performed even if only one end of the channel is connected to active equipment. Regardless of the type of measurement, a transfer function is typically used to describe the relation between the test signal and its response.
The signal may be transmitted using different technologies, e.g. in time domain by sending and collecting pulsed signals (Time-Domain Transmissometry (TDT), Time-Domain Reflectometry (TDR)), or in frequency domain by using wideband, noise-like signals (Frequency-Domain Transmissometry (FDT), Frequency-Domain Reflectometry (FDR). Absence of strong harmful peaks in noise-like signals typically used in FDR/FDT is one of the reasons why FDR/FDT normally is used in above described methods of channel characterization.
A typical FDR/FDT measurement consists of the following steps:    1. Start transmission of periodic multi-carrier test signal X(f)    2. Wait until any transients have decayed sufficiently (e.g. a number of signal periods)    3. Measure reflection or transmission response Y(f) for current signal period    4. Repeat step 4 and average over Navg signal periods to get a mean value of Y(f)    5. Calculate normalized response (estimate of transfer function) by dividing the obtained mean value of Y(f) with X(f)
DSL (Digital Subscriber Line) is a family of technologies providing digital data transmission over wires (transmission lines) of a local network. xDSL is a generic term used to denominate any DSL-technology such as ADSL2 or VDSL2. Methods deployed in order to characterize lines carrying xDSL-traffic are frequently referred to as Double-Ended Line Test (DELT), when corresponding transmitter and receiver are connected to different ends of a transmission line (transmission measurement), and Single-Ended Line Test (SELT), when corresponding transmitter and receiver are connected to the same end of a transmission line (reflection measurement). These methods are in xDSL applications typically performed in frequency domain (FDT, FDR), since xDSL is based on frequency-domain multi-carrier modulation (Discrete Multi-Tone, DMT). Said measurements may be performed using a transceiver that is already a part of the network or a dedicated piece of hardware.
For SELT-measurements, any type of signal may in principle be used as there are no interoperability issues between transmitter and receiver since these are in the same node. Accordingly, signal type is not standardized for SELT-measurements. However, there may exist regulatory limitations (e.g. egress levels) that narrow the choice of signal somewhat.
For DELT-measurements, on the other hand, transmitter and receiver are typically located in different nodes and therefore it is necessary to standardize the signal type so that DELT measurements can be performed between nodes from different vendors.
Consequently, for both SELT and DELT, the signal used for characterizing the line needs to be chosen with care. More specifically, said signal should preferably be periodic as this simplifies averaging of the measurement, i.e. the transfer function can be calculated by frequency-domain division with the transmitted signal once averaging is completed. Further, a periodic signal means that no cyclic prefix needs to be inserted in order to avoid spectral leakage between different subcarriers in the signal. Considering all of the above, one possible type of signal is a so called REVERB-signal. Use of the REVERB-signal for line characterization purposes is also suggested in US 2006/0120442.
Results of measurements, both SELT and DELT, performed using, for instance, the REVERB signal, with purpose to characterize the channel are inevitably degraded under influence of noise. This noise may originate from the transmission line itself or it may be created in the measurement equipment. The term noise includes here all kinds of disturbances that are not dependent on the measurement signal, e.g. thermal noise, crosstalk from other signals and external noise at the receiver end.
In this context, as it is impossible to completely remove noise from a measurement system, a concept of noise floor is introduced. Accordingly, noise floor is a measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system. Noise floor limits the measurement reach since it masks weak (distant) signals.
Methods are known in the art of mitigating reach-limitations. More specifically, in the post-processing phase of a SELT-measurement, reach may be limited by the slowly decaying tail from the near-end echo that hides a weak far-end echo. Solution to this problem is proposed in U.S. Pat. No. 5,461,318.
Moreover, increased measurement time (averaging) can contribute in overcoming reach limitations caused by noise. In particular, for xDSL DELT (also known as Loop Diagnostics), the averaging time is increased compared with the same type of measurements performed during xDSL initialization. This improves somewhat precision of the measurement, e.g. the precision of the measured transfer function.
Increased output power is a simple solution to improve reach of a signal measurement. However, given regulatory aspects and output power limitations, this is not a viable way forward.